1. Field of the Invention
The present invention relates to a manufacturing method of a color wheel suitable for use as a filter element of a time-share light dispersing device, and to a color wheel fabricated by the manufacturing method and incorporated in a color wheel assembly making up a projection-type image display apparatus.
2. Description of the Related Art
Color composition in a projection-type image display apparatus has conventionally been accomplished commonly by a method, such as: a single-panel method, in which one light valve element adapted to control light amount per pixel thereby creating an image is used to disperse each pixel into R (red), G (green), and B (blue) lights; and a three-panel method, in which three light valve elements dedicated to R, G and B lights, respectively, are used to produce in parallel R, G and B images, and then the three images thus produced are composed. Recently, as a light valve element capable of fast switching, such as a ferroelectric liquid crystal display element or a digital micro mirror device, is increasingly coming into practical use, a time-sharing single-panel method is widely used. In the time-sharing single-panel method, R, G and B lights are caused to sequentially impinge on one light valve element, the light valve element is driven in synchronization with switching-over of the R, G and B lights thereby producing R, G and B images in a time-series manner, and the images thus produced are projected onto a screen, or the like. Here, color composition of the images is accomplished by a viewer due to an afterimage effect occurring at a sense of vision. In the time-sharing single-panel method, reduction in both dimension and weight of the apparatus, which is a feature of a single-panel method, can be achieved by employing a relatively simple optical system, and therefore the time-sharing single-panel method is favorable for realizing inexpensive fabrication of a projection-type image display apparatus. In such an image display apparatus, a color wheel is preferably used as a filter element of a time-share light dispersing device to sequentially disperse light emitted from a white light source into R, G and B lights having respective wavelength bands in a time-sharing manner.
FIGS. 18A and 18B are respectively front and side views of a typical color wheel assembly 200 provided with such a color wheel. Referring to FIG. 18B, the color wheel assembly 200 comprises a color wheel 100, a hub 105, and a motor 106. The color wheel 100 is a tricolor color wheel composed of a disk-like substrate 101 which is made of a light-transmitting material, for example, optical glass, and three filter sectors 102, 103 and 104 which are formed on a surface of the substrate 101, and which transmit exclusively, for example, R, G and B lights, respectively. The color wheel 100 thus structured is fixedly attached to the motor 106 via the hub 105 coaxially therewith. The color wheel assembly 200 operates such that the color wheel 100 is rotated by the motor 106 so that the filter sectors (R, G and B) 102, 103 and 104 sequentially have white light S falling incident thereon whereby the white light S is sequentially dispersed into R, G and B lights.
The filter sectors 102, 103 and 104 are usually constituted by optical interference filters of a dielectric multi-layer film structured such that a dielectric thin film formed of a material having a high refractive index (e.g., TiO2, ZrO2, and ZnS), and a dielectric thin film formed of a material having a low refractive index (e.g., SiO2, and MgF2) are alternately laminated by an evaporation method, a sputtering method, or the like. The optical interference filter is superior in durability (heat resistance, light stability, and chemical resistance) to a color filter formed by a staining method, a pigment dispersion method, or the like, has a high transmittance, and easily achieves a sharp spectroscopic characteristic, and therefore endures exposure to intensive light flux and produces a display image of a high visual quality.
A so-called lift-off method is one method for forming such a dielectric multi-layer film with its film formation region precisely demarcated. In the lift-off method, a mask pattern, which is made of a resist film or a metal thin film such that only a predetermined film formation region is exposed, is first formed on a substrate using a prescribed photo processing, and a plating processing as required, then a thin film is formed entirely over the mask pattern, and the mask pattern is fused to be removed thereby lifting off the thin film formed on the mask pattern, thus retaining the thin film only at the predetermined film formation region. This method allows a dielectric multi-layer film to be formed while a film formation region is precisely demarcated by a photo processing without using a troublesome etching processing on a dielectric multi-layer film.
However, the following two problems are found in using the above-described lift-off method for fabricating a color wheel.
Firstly, unlike semiconductor fabrication in which a minute pattern is formed on a substrate, a region to be lifted off has a large area. Specifically, in case of the aforementioned color wheel 100 shown in FIG. 18A, for example when forming the filter sector 102, a region corresponding to a region continuously covered by the filter sectors 103 and 104 is to be lifted off. In this case, since a dielectric multi-layer film is formed also on the entire surface of a mask pattern, resist removing or etching liquid for removing the mask pattern is allowed to effectively penetrate into the mask pattern only from the sidewalls of the pattern mask, which define the outer circumferences of the region continuously covered by the filter sectors 103 and 104, and which usually have a thickness of μm order thus providing an extremely small area as compared to the area of the region to be lifted off. Consequently, an increased time is required for the resist removing or etching liquid to penetrate into the lift-off region and to remove the mask pattern completely, and also it can happen that the mask pattern film once stripped off adheres back to the substrate during the increased time. This leads to deterioration in both working efficiency and product quality. And, in case of a mask pattern made of a resist film, since the resist film is denatured by heat, plasma or ion irradiation during the film formation process, the removing work is made further difficult, which may result in requiring, in addition to the aforementioned increased time, a process performed at a high temperature, or a special treatment, such as high-pressure spraying, ultrasonication, and the like.
Secondly, in the case of color wheel fabrication, since the dimension of a color wheel is determined by the dimension of a substrate, the substrate is minimized for the purpose of downsizing the color wheel, and therefore filter sectors, which are desired to have a maximum possible area, are usually formed fully up to the very periphery of the minimized substrate. The case is different for semiconductor fabrication, where a substrate, which is to be finally cut for yielding plural elements, is so structured as to include at its periphery a blank area with no elements formed, and the blank area is used for holding the substrate inside a film forming apparatus during the film formation process (refer to, for example, Japanese Patent Application Laid-Open No. H05-90391). On the other hand, in the color wheel fabrication, the substrate has no blank area as described above, and it is difficult to hold the substrate inside a film formation apparatus.
Under the circumstances described above, for the purpose of demarcating the filter sectors, the color wheel has conventionally been often fabricated by using a masking jig made of, for example, a thin metal plate (hereinafter referred to as “metal mask”) as disclosed in, for example, Japanese Patent Application Laid-Open No. 2003-57424. In the color wheel fabrication, while minute patterning is not required for forming any of filter sectors, two adjacent filter sectors must abut each other precisely in order to prevent light, which passes a boundary between the two adjacent filter sectors, from failing to determine its color only to become unutilized for image formation. In this regard, the demarcation accomplished by means of a metal mask, in which alignment is principally performed mechanically and secondarily performed visually, is limited in alignment accuracy, and is not capable of achieving the degree of accuracy that is achieved by a photo processing technique used for forming a mask pattern in a lift-off method.